1. Field of the Invention
This invention relates to wireless communication and more particularly to an apparatus that receives transmission of data synchronized to a clock that is out of phase with the clock of the apparatus.
This invention also relates to microprocessor system architecture, and more particularly to a microprocessor architecture that allows read-only memory (ROM) microcode to be effectively altered or enhanced without the need to reprogram or replace the ROM.
2. Description of Related Art
Bluetooth is the name of a well known and open technical specification for wireless communication of data and voice. Bluetooth allows the replacement of very short range cables (wires) now used to connect one device to another with one universal short-range radio link; for example, a computer can thereby communicate with its printer via a radio link instead of a cable. Bluetooth is characterized by operation in the 2.4 GHz and at variable transmission power depending on the distance between devices. For example, for a power class 2 Bluetooth device, the transmission power ranges from 1 mW to 2.5 mW. Bluetooth also allows computing devices to connect to a communicating device via a radio link. For example, a computer can communicate with a nearby cell phone via a radio link to access the Internet. Bluetooth units (radios) connect to each other in “piconets”, which are formed by a master unit connecting up to seven slave units. In a piconet, the slave units are synchronized to the master unit's clock and hopping sequence. See “Specification of the Bluetooth System-Core v 1.0b” available from the Bluetooth Special Interest Group at its web site.
A typical Bluetooth conforming radio includes an RF (radio frequency) circuit and a baseband circuit. The RF circuit receives a frequency modulated electromagnetic signal (i.e., FM signal) carrying data synchronized to the clock signal (“external clock”) of another Bluetooth radio. The baseband circuit includes a demodulator circuit that extracts the data and the external clock signal from the FM signal (symbol timing recovery). The baseband circuit also includes a PLL (phase lock loop) circuit that synchronizes the phase of the internal clock of the baseband circuit to the phase of the external clock to allow data processing circuits of the baseband to process the extracted data. The PLL circuit is necessary because even if the internal clock and the external clock may have substantially the same frequency, they may not have the same phase. Furthermore, interference to the FM signal as it travels through the air may also distort the representation of the frequency of the external clock in the FM signal.
A microprocessor is a central processing unit (CPU) enclosed in one integrated circuit (IC) package. Because of their small size and low cost, microprocessors have revolutionized digital system design technology, giving the designer the capability to create structures that were previously uneconomical. Microprocessors are used in a wide variety of applications. They can function as a CPU in a general purpose computer or as a processor unit in a special purpose, automated system.
Microprocessor systems usually incorporate several different types of memory devices to hold data for processing and instructions for system control. Memory devices come in a wide variety of forms. Two of the more common forms are generally referred to as “read-only memory” (ROM) and “random access memory” (RAM). In its simplest form, sometimes called “mask-programmed”, a ROM memory device is manufactured with fixed contents. The binary information stored in the ROM is made permanent during the hardware production of the unit and cannot subsequently be altered. Such a ROM memory unit performs the read operation only; it does not have a write capability. ROM is most often used to hold microcode, the lowest level instructions that directly control a microprocessor.
By contrast, a RAM is a data storage device that can perform both the read and write function. A system CPU uses its RAM as a storage area for data, calculation results, and program instructions, drawing on this storage as necessary to perform the tasks required by programs. Its binary contents can easily be changed during operation and its stored data is typically erased or lost when the device incorporating it is powered off. Part of the initial “power-up” or boot routine for microprocessor systems includes the loading of desired or necessary code into RAM according to the system design.
Microprocessor systems may also incorporate some type of “non-volatile” memory, such as a hard-disk. Like RAM, such memory can usually be read or written to, but unlike RAM its content is preserved until over-written, even without system power.
More recent varieties of ROM provide for some limited programming capability after manufacture. PROM (“programmable read-only memory”) devices can be programmed by the end user, but only once. EPROM (“erasable programmable read-only memory”) can be erased and then reprogrammed, but must be removed from the system to do so. EEPROM (“electronic erasable read-only memory”) can be erased and programmed electrically some limited number of times with standard supply voltages while in the circuit. However, such devices come with increased cost and limitations of their own. For example, EPROM must be removed from the system to be reprogrammed, and EEPROM must be erased and reprogrammed in its entirety, not selectively. For many processing systems, especially those used to perform set, repetitive tasks, it is desirable to use the cheapest and simplest (mask-programmed) ROM, with minimal reliance on more expensive RAM or other types of memory devices.
However, in even the simplest microprocessor system it may be desirable to alter the execution of the ROM microcode to, for example, correct errors in the code or add new capabilities to it. The disclosed invention allows for alterations in the processing of ROM microcode without incurring the added expense and complexity of integrating some form of programmable ROM into the system.